1. Field of the Invention
The present invention relates generally to a method and apparatus for communication, and more particularly, to a method for sample timing adjustment and frequency offset estimation and compensation, and to a radio system having sample timing adjustment means and frequency offset estimation and compensation means.
2. Description of the Related Art
The present invention relates generally to signal recovery in communication systems and is particularly applicable to, and is described below in the context of, a digital cellular communication system such as the North American TDMA (Time Division Multiple Access) cellular communication system compatible with EIA/TIA documents IS-54 (Revs. A and B) and IS-136.
A conventional wireless radio system used for telephony consists of three basic elementsxe2x80x94namely, mobile units, cell sites, and a Mobile Switching Center (xe2x80x9cMSCxe2x80x9d). In a basic cellular system, a geographic service area, such as a city, is subdivided into a plurality of smaller radio coverage areas, or xe2x80x9ccellsxe2x80x9d. A mobile unit communicates by radio frequency (RF) signals to the cell site within its radio coverage area. The cell site""s base station converts these radio signals for transfer to the MSC via wire (landline) or wireless (microwave) communication links. The MSC routes the call to another mobile unit in the system or the appropriate landline facility. These three elements are integrated to form a ubiquitous coverage radio system that can connect to the public switched telephone network (PSTN).
A mobile unit contains a radio transceiver, a user interface portion, and an antenna assembly, in one physical package. The radio transceiver converts audio to a radio frequency signal for transmission to a cell site and converts received RF signals into audio. The user interface portion includes the display and keypad which allow the subscriber to communicate commands to the transceiver. The antenna assembly couples RF energy between the electronics within the mobile unit and the xe2x80x9cchannelxe2x80x9d, which is the outside air, for transmission and reception. Each mobile unit has a Mobile Identification Number (xe2x80x9cMINxe2x80x9d) stored in an internal memory referred to as a Number Assignment Module (NAM).
A cell site links the mobile unit and the cellular system switching center, and contains a base station, transmission tower, and antenna assembly. The base station converts the radio signals to electrical signals for transfer to and from a switching center.
Digital cellular technology, in which information consisting of voice and data is digitally encoded onto an RF carrier signal, or systems which are compatible with digital and analog cellular communication standards are currently more popular than analog systems. Presently, there are three basic types of digital cellular technology; Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). Digital cellular systems currently fall within these three categories and many use a combination of one or more of these technologies along with analog techniques.
In order to satisfy a demand for a tenfold increase in system capacity over conventional analog cellular systems, the telephone industry group (TIA) of the Electronics Industry Association (EIA/TIA) promulgated an Interim Standard for time division multiplexed (TDM) wireless digital telephony in the late 1980""s, known as IS-54. IS-54 (revs. A and B), and the more current Interim Standard for time division multiplexed wireless telephony, IS-136, use Time Division Multiple Access (TDMA) digital technology to effectively increase the limited bandwidth available for cellular communications. The EIA/TIA IS-54 (revs. A and B) and IS-136 standards are well known in the art and are incorporated herein by reference.
In a TDMA system under IS-54, for instance, data is communicated in symbol bursts arranged in time slots each comprising 162 symbols which include a sync (synchronization) word of 14 symbols followed by an information sequence. Communication from a cell site to mobile units is performed on a time division multiplexed basis whereby each cellular channel is used within each cell to facilitate simultaneous communication with 3 to 6 mobile units. Typically 3 to 6 users (data channels) share a common 30 kHz channel in TDMA operation. Each user transmits data in an assigned time slot that is part of a larger frame. The sync word is used to facilitate timing recovery, i.e., to determine an optimum time for sampling the received signal for further processing to recover the communicated information. It is well known that timing recovery and the necessary processing of the samples are made more difficult by a low signal-to-noise ratio (SNR) and that a low SNR can often be present in cellular communication systems.
In order to receive a transmitted digital signal, the communicating units must determine the beginning and end of signals intended for them, known as frame/slot synchronization. The complexity and accuracy of frame/slot synchronization depend upon the number of points at which the signal is sampled and the ability of the system to compensate for signal distortion. Increasing the number of samples per transmitted symbol results in an increase in the accuracy of the receiver at the expense of a higher complexity. Another crucial function required in TDMA communication is the need to determine the optimum time within a symbol interval to sample the signal and determine the relative phase angle of the symbol. Once the optimum point for symbol timing is determined, all the symbols within a burst can be demodulated using carrier recovery circuitry and the burst decoded and converted into an analog speech signal or other data.
Generally, a received signal is oversampled at N times the symbol rate, wherein N is an integer. Thus N sets of samples of a received signal are stored in a decision block, and various known techniques for determining the optimum set of samples may be applied to extract the information contained in the received signal.
Various methods for determining optimum sampling timing are known, and some such methods determine, on a burst-by-burst basis, the optimum sampling timing by selecting a set of samples which exhibit the best correlation with the previously-known sync word. After determination of an initial optimum sampling timing through timing recovery using the sync word, however, it is necessary to maintain an optimum sampling timing throughout the entire information sequence. This is typically referred to as timing tracking or sample timing adjustment, and serves to avoid cumulative errors of the sampling times during the information sequence, which, if not corrected, can detract from the recovery of the communicated information. Determination of an optimum sampling timing using the sync word does not compensate for signal distortion occurring in a portion of a signal burst separate from the sync word. There is thus a need for a sampling timing adjustment method which is capable of determining optimum sampling timing in a high-loss environment, such as a cellular communication channel, which is sufficiently rapid to effect frequent, high-speed sampling timing adjustments without knowledge of an information sequence in multiplexed or burst communications, such as TDMA cellular communications, and which has minimal processing overhead and hardware requirements.
In a TDMA cellular system, channel-induced signal distortion often appears as a phase shift induced between encoded symbols in a received signal. Since phase modulation of a carrier is used to encode information in TDMA cellular systems, unwanted phase-shifts in a modulated signal may render a signal undetectable. By itself, sampling timing adjustment may not adequately compensate for unwanted phase shift. In addition to determining an optimum sampling timing, therefore, techniques for reducing or eliminating unwanted phase shifts are required in a TDMA cellular system.
In time division multiplexed digital communication systems such as the North American TDMA cellular telephone system, information is typically transmitted as symbols encoded in the phase of the transmitted signal with respect to its carrier. In order to ensure proper extraction of the symbols using coherent detection, the local oscillator frequency used to demodulate the received signal must either be identical to the carrier frequency of the received signal or frequency compensation must be performed on the downconverted signal. Absent compensation for a frequency difference between the carrier of a modulated signal and the local oscillator used to extract the modulated information, the apparent phase relationship xe2x80x9crotatesxe2x80x9d undesirably. Typical methods for the elimination of phase rotation include matching the local oscillator and carrier frequencies, adjusting the frequency of the downconverted signal by an appropriate value, and reconstructing a clock signal used for sampling a downconverted signal based upon the frequency of the carrier.
As is well known in the art, many communication systems, including TDMA cellular systems, rely upon the use of synchronous or so-called xe2x80x9cquasi-synchronousxe2x80x9d detection to extract encoded voice or data contained in a received information signal. In a synchronous detector, the received signal is typically mixed (i.e., multiplied) with a xe2x80x9clocal oscillatorxe2x80x9d signal having a frequency that is matched to the carrier frequency of the received signal. The local oscillator frequency is xe2x80x9clockedxe2x80x9d to the carrier frequency of the received signal to eliminate frequency offset therebetween. In quasi-synchronous detection, a detector that is not locked to the carrier frequency of the received signal is used. Frequency offset is corrected by adjustment of the frequency of the downconverted intermediate frequency (IF) or baseband signal by a frequency offset multiplier having a value based upon a detected or estimated frequency offset.
In certain transmission protocols, the effect of phase rotation is somewhat reduced. In differential quadrature phase-shift keying (xe2x80x9cDQPSKxe2x80x9d), for instance, the encoded information is contained in the difference in phase between a given symbol and the previous symbol, rather than in the absolute phase of the modulated symbol. In an ideal channel, a frequency offset between the local oscillator of the receiver and the carrier frequency of the transmitted signal does not present a significant problem in a system employing DQPSK as long as the symbol frequency is much larger than the frequency offset.
The cellular channel is not ideal, however, and is subject to various types of distortion such as delay spread due to multipath fading, the Doppler effect, flat and frequency-selective fading, additive noise, and the like Thus, a process such as adaptive equalization, which involves the adaptive characterization of channel distortion, is needed in order to extract symbols accurately from the time-dispersive channel. To estimate and compensate for channel-induced distortion, cellular systems typically utilize adaptive equalization techniques which predict the channel response based upon the transmission of known data (e.g., a so-called training sequence) However, such processes are themselves sensitive to significant distortion, including frequency offsets, which may cause the channel to vary beyond the rate at which the adaptive processes can adapt. Even for DQPSK systems, therefore, accurate sample timing adjustment and frequency offset compensation is necessary.
Many conventional sampling timing adjustment and frequency offset estimation methods utilize the sync word or other known information sequence to effect compensation for signal distortion. In a time-dispersive channel, however, it would be desirable to perform sampling timing adjustment and frequency offset correction to compensate for signal distortion occurring in a portion of a signal burst separate from a sync word or other predetermined information sequence. Such errors may result in an increased bit error rate when optimum sampling timing and frequency offset estimation is determined based solely upon the known information sequence.
Accordingly, there is a need for a sufficiently rapid and simplified method and apparatus for sampling timing adjustment and frequency offset estimation which may be used with known or unknown data and does not require prior knowledge of an encoded information sequence, and which minimizes processing time and optimally avoids the need for costly additional circuitry.
While current methods used for the reduction or elimination of phase shift due to channel-induced distortion have the tendency to increase decoding accuracy and reduce the need for repeated sampling timing adjustment, conventional phase correction techniques may not meet the demand for the reduced size and cost requirements of a particular system. Thus, a simplified correction technique for enhancing, supplementing or replacing known frequency tracking and sampling timing adjustment means is needed.
For instance, a conventional technique used for phase tracking employs a phase-locked-loop (xe2x80x9cPLLxe2x80x9d). A PLL circuit is typically formed as a phase detector fed by input and feedback signals, a loop filter and a voltage controlled oscillator for producing a sine wave (i.e., the feedback signal). The phase of the received signal, or a frequency-translated version thereof (e.g., an intermediate frequency (IF) signal), is compared with the local phase reference (i.e., the local oscillator), and the average phase difference over time is used to adjust the frequency of the reference. Unfortunately, PLL systems tend to require a fair amount of time to achieve phase lock, and the result in a cellular telephone system can be unacceptable. In addition, in cellular systems based on burst data transfer, control data is often contained in a single packet, which may be lost before phase lock is achieved. An objectionable amount of dead time may also be encountered during handoff from one cell to another. This is true both for conventional, analog PLLs and for digital equivalents. Moreover, in wireless communications, the design of automatic frequency control (AFC) circuitry, such as PLLS, has been constrained by circuit complexity, and system designs have typically made frequency accuracy constraints somewhat loose to avoid prohibitive costs in complexity or processing requirements. There is thus a need for sampling timing adjustment and frequency offset compensation methods which are capable of enhancing, supplementing or replacing PLL circuits which are not capable of achieving adequate phase lock in a given communication system.
In addition, with the introduction of more optimal modulation schemes such as QPSK, relatively precise frequency estimates are often needed. Frequency errors may arise, for example, from the transmitter/receiver clock not being perfectly locked due to inaccuracies or drift in the crystal oscillator, as well as from large frequency shifts due to the Doppler effect, such as those occurring from vehicles moving at high speeds in open spaces. Many cellular systems allow only a small amount of time for achieving initial signal acquisition and require a minimum tracking error after initial acquisition. However, typical AFC or PLL circuits are not generally able to lock on or track the received signal with a reasonable degree of accuracy.
The extraction of encoded information from a signal transmitted over a time-varying cellular channel thus requires a plurality of processes for attaining accurate frame/slot synchronization, optimum sampling timing adjustment, and frequency offset estimation. It would be desirable to minimize the structure and processing required to perform these processes.
In application Ser. No. 09/371529, assigned to the assignee herein, and entitled xe2x80x9cMethod and Apparatus for Frequency Offset Compensationxe2x80x9d, applicants disclose a technique for frequency offset estimation and compensation which relies on differential phase information extracted from the modulated signal in order to estimate the frequency offset. This process monitors and corrects for deviations from ideal values in the differential phase angle between successively received symbol samples in a DQPSK-modulated waveform, Since frequency offset results in a fairly constant phase offset in a received waveform over an entire burst of symbols, detection of a constant deviation in phase angle over a predetermined number of symbols provides a sufficiently accurate estimate of frequency offset. Determination of non-constant deviations from ideal conditions is also possible, thus providing a system which is capable of achieving a desired bit error rate.
The need to perform various signal enhancement techniques such as sampling timing adjustment and frequency offset correction to reduce the bit error rate in a communication system increases the hardware requirements and processing throughput of such a system. There is thus a need for a technique which reduces the hardware and processing requirements required to perform these processes.
The present invention is based upon the recognition that certain modulation techniques exhibit statistical characteristics that are detectable even in the presence of severe channel-induced distortion. The time and processing requirements needed to perform sampling timing adjustment can be substantially reduced by determining an optimum sampling timing based upon a minimum deviation from expected differential phase angles between successive pairs of received symbols in an ideal channel. By detecting an offset from expected differential phase angles ideal conditions, i.e., phase rotation, frequency offset can also be estimated based upon the same differential phase values calculated for sampling timing adjustment purposes, thereby reducing the necessary hardware and processing requirements for performing sampling timing adjustment and frequency offset estimation.
In xcfx80/4-shifted DQPSK, the possible differential phase angles between two consecutive received symbols over an ideal channel are xc2x145xc2x0 or xc2x1135xc2x0. Channel effects produce results which deviate from ideal conditions. However, even in the presence of severe channel distortion there is a statistical concentration of differential phase angle values in the vicinity of xc2x145xc2x0 or xc2x1135xc2x0. By determining which of a plurality of sets of samples of an oversampled signal has differential phase angles between successive symbols which are closest to the ideal values, the optimal sampling timing can be determined.
In accordance with one aspect of the present invention, sampling timing adjustment is performed in a TDMA cellular system by oversampling a received signal to produce a plurality of sets of samples of the signal, determining for each set of samples the differential phase angle between successive symbols by multiplying pairs of successively received symbols to produce a vector having an angle representing the phase angle therebetween, and determining which set of samples has differential phase angles closest to the expected values under ideal conditions.
In order to correct for frequency offset, a deviation in differential phase angle is detected from a selected set of samples. As noted above, the expected differential phase angle between successive samples in xcfx80/4-shifted DQPSK, in an ideal channel, is either xc2x145xc2x0 or xc2x1135xc2x0. A deviation from the expected phase angles is observed, and this deviation is indicative of xe2x80x9cphase rotationxe2x80x9d due to frequency offset between the local reference and the carrier. This phase rotation can then be used to adjust the local oscillator frequency. Since this approach eliminates the need to average over long periods, the filtering out of data-dependent effects, and the use of known data sequences, the time required to achieve adequate frequency-offset compensation is shorter in many important environments than it is for conventional systems.
By conducting similar processing for sampling timing adjustment and frequency offset estimation, including the determination of differential phase angles for successive received symbols, shared processing can be performed thereby reducing the hardware and processing requirements needed for sampling timing adjustment and frequency offset estimation.